The revpath program prints out a relative
path that is the ``reverse'' or ``inverse'' of path. Start with two directories
top and bottom, with the latter below the former, and path is the location
of bottom relative to top. The output of revpath is the location of top
relative to bottom. The resulting path contains a trailing `/' character when
the result is non-trivial. If path is equivalent to `.', the resulting output
is empty. If path is invalid in some way (e.g., doesn't represent the path
to a subdirectory) the output is also empty and no error messages are ever
generated.